Motor control apparatus and motor control method

ABSTRACT

In response to detection of a skid of drive wheels based on an increase in angular acceleration α of a rotating shaft of a motor, the control procedure of the invention refers to a map representing a variation in maximum torque Tmax against the angular acceleration α and sets torque restriction of the motor to limit the torque level of the motor to the maximum torque Tmax corresponding to a peak value of the angular acceleration α. When the torque restriction sufficiently lowers the angular acceleration α to detect convergence of the skid, the control procedure cancels the torque restriction to a certain level of the maximum torque Tmax corresponding to a torque restoration limit δ 1 , which is set according to the degree of the skid. The torque restoration limit δ 1  (that is, the maximum torque Tmax) is cancelled in a stepwise manner by a cancellation rate and a cancellation time corresponding to an additional accelerator depression relative to an accelerator opening at the time of detection of the skid. The control procedure sets the greater cancellation rate and the shorter cancellation time with an increase in additional accelerator depression.

This is a 371 national phase application of PCT/JP2003/008593 filled 7Jul. 2003, claiming priority to Japanese Patent Application No.2002-251363 filed 29 Aug. 2002, the contents of which are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a motor control apparatus and a motorcontrol method. More specifically the invention pertains to a motorcontrol apparatus that controls a motor, which is mounted on a vehicleand outputs power to a drive shaft linked to drive wheels, as well as toa corresponding motor control method.

BACKGROUND ART

One proposed motor control apparatus restricts torque output from amotor to a drive shaft, in response to occurrence of a skid due towheelspin of drive wheels with the torque output from the motor (see,for example, Japanese Patent Laid-Open Gazette No. 10-304514). Thismotor control apparatus restricts the torque level output from the motorin response to detection of a skid based on an increase in angularacceleration of the drive wheels (that is, a time variation of angularvelocity), while canceling the torque restriction of the motor inresponse to elimination of the skid by the torque restriction.

This prior art motor control apparatus uniformly cancels the torquerestriction, regardless of the driver's demand. This may cause thedriver to feel uncomfortable and worsen the drivability.

The applicant of the present invention has disclosed a vehicle skidcontrol technique that regulates a degree of torque restriction, whichis set in response to occurrence of a skid, according to an acceleratoropening or a driver's step-on amount of an accelerator pedal andregulates a degree of cancellation of the torque restriction in responseto elimination of the skid (see Japanese Patent Laid-Open Gazette No.2001-295676).

DISCLOSURE OF THE INVENTION

The motor control apparatus and the corresponding motor control methodof the invention aim to enhance drivability in skid control of avehicle. The motor control apparatus and the corresponding motor controlmethod of the invention also aim to prevent an excessive skid of avehicle while reflecting a driver's acceleration demand in skid controlof the vehicle.

At least part of the above and the other related objects is attained bythe motor control apparatus and the corresponding motor control methodof the invention having the arrangements discussed below.

A motor control apparatus of the invention controls a motor, which ismounted on a vehicle and outputs power to a drive shaft linked to drivewheels, and includes: a skid detection module that detects a skid due towheelspin of the drive wheels; a torque restriction control module that,in response to detection of a skid by the skid detection module, setstorque restriction for reduction of the skid and controls the motorunder the torque restriction; and a torque restriction cancellationcontrol module that, in response to at least a reducing tendency of theskid, cancels the torque restriction, which is set by the torquerestriction control module, to a specific degree corresponding to avariation in driver's accelerator operation, and controls the motorunder at least partly cancelled torque restriction.

The motor control apparatus of the invention detects a skid due towheelspin of the drive wheels, sets torque restriction for reducing theskid in response to detection of a skid, and controls the motor underthe torque restriction. In response to at least a reducing tendency ofthe skid, the motor control apparatus cancels the torque restriction toa specific degree corresponding to a variation in driver's acceleratoroperation, and controls the motor under at least partly cancelled torquerestriction. The cancellation of the torque restriction in response tothe reducing tendency of the skid reflects the variation in driver'saccelerator operation, that is, the driver's acceleration demand of thevehicle under the condition of occurrence of a skid. This arrangementdesirably enhances the drivability in cancellation of the torquerestriction, compared with a prior art arrangement that does not reflectthe driver's acceleration demand in cancellation of the torquerestriction.

In the motor control apparatus of the invention, the variation indriver's accelerator operation may represent a rate of change relativeto a reference accelerator operation at a time of detection of a skid bythe skid detection module. This arrangement adequately understands thedriver's acceleration demand of the vehicle under the condition ofoccurrence of a skid.

In the motor control apparatus of the invention, the torque restrictioncancellation control module may cancel the torque restriction in astepwise manner with elapse of time. This arrangement desirably lowersthe potential for reoccurrence of a skid by cancellation of the torquerestriction. In this embodiment of the motor control apparatus, thetorque restriction cancellation control module may control the motorwith a tendency of increasing a cancellation rate of the torquerestriction with an increase in driver's additional depression of anaccelerator pedal as the variation in driver's accelerator operation.This arrangement cancels the torque restriction by a greatercancellation rate corresponding to the driver's acceleration demand.Further, in this embodiment of the motor control apparatus, the torquerestriction cancellation control module may control the motor with atendency of shortening a cancellation time of the torque restrictionwith an increase in driver's additional depression of an acceleratorpedal as the variation in driver's accelerator operation. Thisarrangement cancels the torque restriction within a shorter cancellationtime corresponding to the driver's acceleration demand.

The motor control apparatus of the invention may further include: anangular acceleration measurement module that measures an angularacceleration of either of the drive shaft and a rotating shaft of themotor, and in this embodiment, the skid detection module may detect askid, based on a variation in measured angular acceleration, and thetorque restriction control module, in response to detection of a skid,may change a degree of the torque restriction corresponding to theangular acceleration measured by the angular acceleration measurementmodule and controls the motor under the changed degree of the torquerestriction. This arrangement effectively sets the torque restrictionaccording to the degree of the skid, which is based on the angularacceleration, so as to reduce the skid.

In the motor control apparatus of the invention, the vehicle may havedriven wheels that are driven by the drive wheels, and the motor controlapparatus may further include: a drive wheel rotation speed measurementmodule that measures a rotation speed of the drive wheels; and a drivenwheel rotation speed measurement module that measures a rotation speedof the driven wheels. In this embodiment, the skid detection module maydetect a skid, based on a rotation speed difference between the rotationspeed of the drive wheels measured by the drive wheel rotation speedmeasurement module and the rotation speed of the driven wheels measuredby the driven wheel rotation speed measurement module, and the torquerestriction control module, in response to detection of a skid, maychange a degree of the torque restriction corresponding to the rotationspeed difference and control the motor under the changed degree of thetorque restriction. This arrangement effectively sets the torquerestriction according to the degree of the skid, which is based on therotation speed difference between the rotation speed of the drive wheelsand the rotation speed of the driven wheels, so as to reduce the skid.

In the motor control apparatus of the invention, the motor controlapparatus may further include: a torque re-restriction control modulethat, in response to detection of another skid by the skid detectionmodule under control of the motor by the torque restriction cancellationcontrol module, sets torque re-restriction for reduction of the anotherskid and controls the motor under the torque re-restriction. Thisarrangement effectively reduces another skid occurring undercancellation of the torque restriction corresponding to the variation indriver's accelerator operation. The motor control apparatus of theinvention structured in this way may further include: an angularacceleration measurement module that measures an angular acceleration ofeither of the drive shaft and a rotating shaft of the motor, and in thisembodiment, the skid detection module may detect a skid, based on avariation in measured angular acceleration, and the torquere-restriction control module, in response to detection of another skidby the skid detection module, may change a degree of the torquere-restriction corresponding to a peak value of the angular accelerationmeasured by the angular acceleration measurement module and control themotor under the changed degree of the torque re-restriction. Thisarrangement effectively re-restricts the torque according to the degreeof another skid, which is based on the peak value of the angularacceleration. The motor control apparatus of the invention may furtherinclude: a torque restriction re-cancellation control module thatcancels the torque re-restriction set by the torque re-restrictioncontrol module after elapse of a preset time period corresponding to avariation in driver's accelerator opening, regardless of state of theanother skid, and controls the motor under the cancelled torquere-restriction. This arrangement responds to the driver's accelerationdemand of the vehicle, while desirably preventing an excess amount ofanother skid.

A motor control method of the invention controls a motor, which ismounted on a vehicle and outputs power to a drive shaft linked to drivewheels, and the motor control method include the steps of: (a) detectinga skid due to wheelspin of the drive wheels; (b) in response todetection of a skid by the step (a), setting torque restriction forreduction of the skid and controlling the motor under the torquerestriction; and (c) in response to at least a reducing tendency of theskid, canceling the torque restriction, which is set by the step (b), toa specific degree corresponding to a variation in driver's acceleratoroperation, and controlling the motor under at least partly cancelledtorque restriction.

In the motor control method of the invention, the variation in driver'saccelerator operation may represent a rate of change relative to areference accelerator operation at a time of detection of a skid by thestep (a).

Further, in the motor control method of the invention, the step (c) maycancel the torque restriction in a stepwise manner with elapse of time.In this embodiment of the motor control method, the step (c) may controlthe motor with a tendency of increasing a cancellation rate of thetorque restriction with an increase in driver's additional depression ofan accelerator pedal as the variation in driver's accelerator operation.Moreover, in the motor control method of the invention, the step (c) maycontrol the motor with a tendency of shortening a cancellation time ofthe torque restriction with an increase in driver's additionaldepression of an accelerator pedal as the variation in driver'saccelerator operation.

The technique of the invention is not restricted to the motor controlapparatus or the corresponding motor control method discussed above, butmay also be actualized by a vehicle equipped with a motor and the motorcontrol apparatus of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the configuration of an electricvehicle 10 equipped with a motor control apparatus 20 in one embodimentof the invention;

FIG. 2 is a flowchart showing a motor drive control routine executed byan electronic control unit 40 in the motor control apparatus 20 of theembodiment;

FIG. 3 is a map showing variations in motor toque demand Tm* againstvehicle speed V and accelerator opening Acc;

FIG. 4 is a flowchart showing a skid state determination routineexecuted by the electronic control unit 40 in the motor controlapparatus 20 of the embodiment;

FIG. 5 is a flowchart showing a skid occurring state control routineexecuted by the electronic control unit 40 in the motor controlapparatus 20 of the embodiment;

FIG. 6 is a map showing a variation in maximum torque Tmax againstangular acceleration α of a motor 12;

FIG. 7 is a flowchart showing a skid convergence state control routineexecuted by the electronic control unit 40 in the motor controlapparatus 20 of the embodiment;

FIG. 8 is a flowchart showing a torque restoration limit δ1 settingroutine executed by the electronic control unit 40 in the motor controlapparatus 20 of the embodiment;

FIG. 9 is a flowchart showing a torque restoration limit δ1 cancellationroutine executed by the electronic control unit 40 in the motor controlapparatus 20 of the embodiment;

FIG. 10 is a map showing variations in cancellation time t againstskid-state accelerator opening Accslip and additional acceleratordepression ΔAcc;

FIG. 11 is a map showing variations in cancellation increment D1 againstthe skid-state accelerator opening Accslip and the additionalaccelerator depression ΔAcc;

FIG. 12 is a flowchart showing a torque restriction rate δsafe settingand cancellation routine executed by the electronic control unit 40 inthe motor control apparatus 20 of the embodiment;

FIG. 13 is a map showing a variation in torque restriction rate δsafeagainst peak value αpeak of the angular acceleration α;

FIG. 14 shows a process of setting the maximum torque Tmax;

FIG. 15 is a flowchart showing a skid state determination routineexecuted by an electronic control unit in a motor control apparatus of asecond embodiment;

FIG. 16 is a flowchart showing a skid occurring state control routineexecuted by an electronic control unit in a motor control apparatus of asecond embodiment;

FIG. 17 is a flowchart showing a torque restriction rate δ2 settingroutine executed by an electronic control unit in a motor controlapparatus of a second embodiment;

FIG. 18 is a flowchart showing a skid convergence state control routineexecuted by an electronic control unit in a motor control apparatus of asecond embodiment;

FIG. 19 is a flowchart showing a torque restriction rate δ2 cancellationroutine executed by an electronic control unit in a motor controlapparatus of a second embodiment;

FIG. 20 shows a process of setting the maximum torque Tmax;

FIG. 21 schematically illustrates the configuration of a hybrid vehicle110;

FIG. 22 schematically illustrates the configuration of a hybrid vehicle210; and,

FIG. 23 schematically illustrates the configuration of a hybrid vehicle310.

BEST MODES OF CARRYING OUT THE INVENTION

Some modes of carrying out the invention are described below aspreferred embodiments. FIG. 1 schematically illustrates theconfiguration of an electric vehicle 10 equipped with a motor controlapparatus 20 in one embodiment of the invention. As illustrated, themotor control apparatus 20 of the embodiment is constructed to drive andcontrol a motor 12, which uses electric power supplied from a battery 16via an inverter circuit 14 and outputs power to a drive shaft linked todrive wheels 18 a, 18 b of the electric vehicle 10. The motor controlapparatus 20 includes a rotation angle sensor 22 that measures arotation angle θ of a rotating shaft of the motor 12, a vehicle speedsensor 24 that measures a driving speed of the vehicle 10, wheel speedsensors 26 a, 26 b, 28 a, and 28 b that respectively measure wheelspeeds of the drive wheels (front wheels) 18 a and 18 b and drivenwheels (rear wheels) 19 a and 19 b driven by the drive wheels 18 a and18 b, diversity of sensors that detect the driver's various operations(for example, a gearshift position sensor 32 that detects thedriver'setting position of a gearshift lever 31, an accelerator pedalposition sensor 34 that detects the driver's step-on amount of anaccelerator pedal 33 (an accelerator opening), and a brake pedalposition sensor 36 that detects the driver's step-on amount of a brakepedal 35 (a brake opening)), and an electronic control unit 40 thatcontrols the respective constituents of the apparatus.

The motor 12 is, for example, a known synchronous motor generator thatfunctions both as a motor and a generator. The inverter circuit 14includes multiple switching elements that convert a supply of electricpower from the battery 16 into another form of electric power suitablefor actuation of the motor 12. The structures of the motor 12 and theinverter circuit 14 are well known in the art and are not the key partof this invention, thus not being described here in detail.

The electronic control unit 40 is constructed as a microprocessorincluding a CPU 42, a ROM 44 that stores processing programs, a RAM 46that temporarily stores data, and input and output ports (not shown).The electronic control unit 40 receives, via the input port, therotation angle θ of the rotating shaft of the motor 12 measured by therotation angle sensor 22, the vehicle speed V of the vehicle 10 measuredby the vehicle speed sensor 24, the wheel speeds Vf1 and Vf2 of thedrive wheels 18 a and 18 b and the wheel speeds Vr1 and Vr2 of thedriven wheels 19 a and 19 b measured by the wheel speed sensors 26 a, 26b, 28 a, and 28 b, the gearshift position detected by the gearshiftposition sensor 32, the accelerator opening Acc detected by theaccelerator pedal position sensor 34, and the brake opening detected bythe brake pedal position sensor 36. The electronic control unit 40outputs control signals, for example, switching control signals to theswitching elements of the inverter circuit 14 to drive and control themotor 12, via the output port.

The description regards the operations of the motor control apparatus 20constructed as discussed above, especially a series of operations ofdriving and controlling the motor 12 in the event of occurrence of askid due to wheelspin of the drive wheels 18 a and 18 b of the vehicle10. FIG. 2 is a flowchart showing a motor drive control routine executedby the electronic control unit 40 in the motor control apparatus 20 ofthe embodiment. This control routine is repeatedly executed at presettime intervals (for example, at every 8 msec).

When the motor drive control routine starts, the CPU 42 of theelectronic control unit 40 first inputs the accelerator opening Acc fromthe accelerator pedal position sensor 34, the vehicle speed V from thevehicle speed sensor 24, wheel speeds Vf and Vr from the wheel speedsensors 26 a, 26 b, 28 a, and 28 b, and a motor rotation speed Nmcalculated from the rotation angle θ measured by the rotation anglesensor 22 (step S100). In this embodiment, the wheel speeds Vf and Vrrespectively represent an average of the wheel speeds Vf1 and Vf2measured by the wheel speed sensors 26 a and 26 b and an average of thewheel speeds Vr1 and Vr2 measured by the wheel speed sensors 28 a and 28b. The vehicle speed V is measured by the vehicle speed sensor 24 inthis embodiment, but may alternatively be calculated from the wheelspeeds Vf1, Vf2, Vr1, and Vr2 measured by the wheel speed sensors 26 a,26 b, 28 a, and 28 b.

The CPU 42 then sets a torque demand Tm* of the motor 12 according tothe input accelerator opening Acc and the input vehicle speed V (stepS102). A concrete procedure of setting the motor torque demand Tm* inthis embodiment stores in advance variations in motor torque demand Tm*against the accelerator opening Acc and the vehicle speed V as a map inthe ROM 44 and reads the motor torque demand Tm* corresponding to thegiven accelerator opening Acc and the given vehicle speed V from themap. One example of this map is shown in FIG. 3.

The CPU 42 subsequently calculates an angular acceleration α from themotor rotation speed Nm input at step S100 (step S104). The calculationof the angular acceleration α in this embodiment subtracts a previousrotation speed Nm input in a previous cycle of this routine from acurrent rotation speed Nm input in the current cycle of this routine(current rotation speed Nm—previous rotation speed Nm). The unit of theangular acceleration α is [rpm/8 msec] since the execution interval ofthis routine is 8 msec in this embodiment, where the rotation speed Nmis expressed by the number of rotations per minute [rpm]. Any othersuitable unit may be adopted for the angular acceleration α as long asthe angular acceleration α is expressible as a time variation ofrotation speed. In order to minimize a potential error, the angularacceleration α may be an average of angular accelerations calculated ina preset number of cycles of this routine (for example, 3).

The CPU 42 determines a skid state of the drive wheels 18 a and 18 bbased on the calculated angular acceleration α (step S106), and executesa required series of control according to the result of thedetermination (steps S110 to S114), before terminating this motor drivecontrol routine. The determination of no occurrence of a skid (when botha skid occurrence flag F1 and a skid convergence flag F2 described beloware set equal to 0) triggers grip-state control (step S110). Thedetermination of the occurrence of a skid (when the flag F1 is set equalto 1 and the flag F2 is set equal to 0) triggers skid occurring statecontrol (step S112). The determination of convergence of a skid (whenboth the flags F1 and F2 are set equal to 1) triggers skid convergencestate control (step S114).

The determination of the skid state follows a skid state determinationroutine shown in FIG. 4. When the skid state determination routinestarts, the CPU 42 of the electronic control unit 40 compares theangular acceleration α calculated at step S104 in the control routine ofFIG. 2 with a preset threshold value αslip, which suggests theoccurrence of a skid due to wheelspin (step S130). When the calculatedangular acceleration α exceeds the preset threshold value αslip, the CPU42 determines the occurrence of a skid on the wheels 18 a and 18 b andsets the value ‘1’ to a skid occurrence flag F1 representing theoccurrence of a skid (step S132), before exiting from this skid statedetermination routine. When the calculated angular acceleration α doesnot exceed the preset threshold value αslip, on the other hand, the CPU42 determines whether the skid occurrence flag F1 is equal to 1 (stepS134). When the skid occurrence flag F1 is equal to 1, the CPU 42subsequently determines whether the calculated angular acceleration αhas been kept negative for a preset time period (step S136). In the caseof an affirmative answer, the CPU 42 determines convergence of the skidoccurring on the drive wheels 18 a and 18 b and sets the value ‘1’ to askid convergence flag F2 (step S138), before exiting from this skidstate determination routine. In the case of a negative answer, on theother hand, the CPU 42 determines no convergence of the skid andterminates this skid state determination routine. When the calculatedangular acceleration α does not exceed the preset threshold value αslipand the skid occurrence flag F1 is not equal to 1, the CPU 42 sets boththe skid occurrence flag F1 and the skid convergence flag F2 equal to 0(step S140) and terminates this skid state determination routine. Therespective controls of the motor 12 according to the values of the skidoccurrence flag F1 and the skid convergence flag F2 are described indetail below.

The grip state control is normal drive control of the motor 12 anddrives and control the motor 12 to ensure output of a torquecorresponding to the preset torque demand Tm*.

The skid occurring state control drives and controls the motor 12 tolower the angular acceleration α, which was increased by the occurrenceof a skid, and follows a skid occurring state control routine of FIG. 5.The CPU 42 of the electronic control unit 40 first compares the angularacceleration α with a preset peak value αpeak (step S150). When theangular acceleration α exceeds the preset peak value αpeak, the peakvalue αpeak is updated to the current value of the angular accelerationα (step S152). The peak value αpeak represents a peak of the angularacceleration α increasing due to a skid and is initially set equal to 0.Until the angular acceleration α increases to reach its maximum, thepeak value αpeak is successively updated to the current value of theangular acceleration α. When the increasing angular acceleration αreaches its maximum, the maximum value of the increasing angularacceleration α is fixed to the peak value αpeak. After setting the peakvalue αpeak, the CPU 42 sets a maximum torque Tmax as an upper limit oftorque output from the motor 12 corresponding to the peak value αpeak(step S154). The procedure of this embodiment refers to a map shown inFIG. 6 to set the maximum torque Tmax. FIG. 6 shows a variation inmaximum torque Tmax against the angular acceleration α. As illustratedin this map, the maximum torque Tmax decreases with an increase inangular acceleration α. The greater peak value αpeak with an increase inangular acceleration α, that is, the heavier skid, sets the smallervalue to the maximum torque Tmax and limits the output torque of themotor 12 to the smaller maximum torque Tmax.

After setting the maximum torque Tmax, the motor torque demand Tm* iscompared with the maximum torque Tmax (step S156). When the motor torquedemand Tm* exceeds the maximum torque Tmax, the motor torque demand Tm*is limited to the maximum torque Tmax (step S158). The CPU 42 then setsthe motor torque demand Tm* to a target torque and drives and controlsthe motor 12 to output a torque corresponding to the target torque Tm*(step S160), before exiting from this skid occurring state controlroutine. The torque output from the motor 12 in the occurrence of a skidis limited to a lower level (that is, the maximum torque Tmaxcorresponding to the peak value αpeak of the angular acceleration in themap of FIG. 6) for immediate reduction of the skid. This limitationeffectively reduces the skid.

The skid convergence state control drives and controls the motor 12 torestore the limited torque level, when the torque restriction by theskid occurring state control lowers the angular acceleration α andconverges the skid. The skid convergence state control follows a skidconvergence state control routine of FIG. 7. The CPU 42 of theelectronic control unit 40 first inputs a torque restoration limit δ1and a torque restriction rate δsafe (both expressed in the same unit[rpm/8 msec] as the angular acceleration) (step S170).

The torque restoration limit δ1 is a parameter used to set a degree ofrestoration from the torque restriction by increasing the maximum torqueTmax, which has been set in the skid occurring state control describedabove. The initial value of the torque restoration limit δ1 is set equalto 0. The torque restoration limit δ1 is set according to a torquerestoration limit δ1 setting routine shown in FIG. 8 as discussed below.The torque restoration limit δ1 setting routine of FIG. 8 is executedwhen the skid occurrence flag F1 is set from 0 to 1 (that is, when thecalculated angular acceleration α exceeds the preset threshold valueαslip) at step S132 in the skid state determination routine of FIG. 4.The CPU 42 of the electronic control unit 40 first inputs the motorrotation speed Nm calculated from the rotation angle θ measured by therotation angle sensor 22 (step S200) and calculates the angularacceleration α of the motor 12 from the input motor rotation speed Nm(step S202). The CPU 42 then integrates the angular acceleration α togive a time integration αint thereof over an integration interval sincethe angular acceleration α exceeded the preset threshold value αslip(step S204). In this embodiment, the time integration αint of theangular acceleration α is given by Equation (1) below, where Δt denotesa time interval of the repeated execution of steps S200 to S204 asdescribed below and is set equal to 8 msec in this embodiment:αint←αint+(α−αslip)·Δt   (1)

The processing of steps S200 to S204 is repeated until the angularacceleration α decreases below the preset threshold value αslip (stepS196). Namely the integration interval is between the time point whenthe angular acceleration α exceeds the threshold value αslip and thetime point when the angular acceleration α decreases below the thresholdvalue αslip. The torque restoration limit δ1 is set by multiplying thetime integration αint by a predetermined coefficient k1 (step S208). Thetorque restoration limit δ1 setting routine is here terminated. Thisroutine calculates the torque restoration limit δ1 by multiplication ofthe predetermined coefficient k1. One modified procedure may prepare inadvance a map representing a variation in torque restoration limit δ1against the time integration αint and read the torque restoration limitδ1 corresponding to the given time integration αint from the map. Thisroutine calculates the torque restoration limit δ1 from the timeintegration of the angular acceleration α. Another modified proceduremay set the torque restoration limit δ1 based on a peak value of theangular acceleration α in the skid occurring state (that is, the valueof the angular acceleration α when the time integration dα/dt of theangular acceleration α is approximate to zero). Still another modifiedprocedure may set a fixed value to the torque restoration limit δ1,irrespective of the angular acceleration α. The concrete process ofsetting the torque restoration limit δ1 writes the value of the torquerestoration limit δ1 into a specific area of the RAM 46.

The torque restriction rate δsafe is a parameter set to reduce anotherskid, which occurs during the repeated execution of the skid convergencestate control routine of FIG. 7. The initial value of the torquerestriction rate δsafe is equal to 0. The torque restriction rate δsafeis described in detail later. As a matter of convenience, the followingdescription regards the skid convergence state control routine of FIG. 7first on the assumption that no other skid occurs (that is, when theinput torque restriction rate δ2 is equal to 0) and then on theassumption that another skid occurs.

After input of the torque restoration limit δ1, the CPU 42 inputs acancellation request of canceling the torque restoration limit δ1 if any(step S172) and determines whether the cancellation request has beenentered (step S174). This process determines whether a cancellationrequest has been input to cancel the torque restoration limit δ1 as theparameter used to set the maximum torque Tmax. The concrete procedure ofinputting a cancellation request reads out the cancellation request,which was written in a predetermined area in the RAM 46 according to atorque restoration limit δ1 cancellation routine of FIG. 9 as discussedbelow. This torque restoration limit δ1 cancellation routine is executedrepeatedly at preset time intervals (for example, at every 8 msec)during execution of the skid convergence state control routine of FIG. 7(while the skid convergence flag F2 is fixed to the value 1).

When the torque restoration limit δ1 cancellation routine starts, theCPU 42 of the electronic control unit 40 first inputs a skid-stateaccelerator opening Accslip and the accelerator opening Acc (step S210).The skid-state accelerator opening Accslip represents an acceleratoropening at the time of the occurrence of a skid. In a more concretedefinition, the skid-state accelerator opening Accslip is an acceleratoropening detected by the accelerator pedal position sensor 34 when theskid occurrence flag F1 is set from 0 to 1. In this embodiment, theconcrete procedure of inputting the skid-state accelerator openingAccslip reads out the accelerator opening, which was detected by theaccelerator pedal position sensor 34 at the time of the occurrence of askid and was written into a predetermined area in the RAM 46. The CPU 42subsequently subtracts the input skid-state accelerator opening Accslipfrom the input accelerator opening Acc to calculate an additionalaccelerator depression ΔAcc (=Acc−Accslip) since the occurrence of theskid (step S212). The CPU 42 sets a cancellation time t of the torquerestoration limit δ1, based on the calculated additional acceleratordepression ΔAcc and the input skid-state accelerator opening Accslip(step S214). A concrete procedure of setting the cancellation time t ofthe torque restoration limit δ1 in this embodiment stores in advancevariations in cancellation time t against the additional acceleratordepression ΔAcc and the skid-state accelerator opening Accslip as a mapin the ROM 44 and reads the cancellation time t corresponding to thegiven additional accelerator depression ΔAcc and the given skid-stateaccelerator opening Accslip from the map. One example of this map isshown in FIG. 10. As shown in FIG. 10, a shorter time period is set tothe cancellation time t with an increase in additional acceleratordepression ΔAcc. The greater additional accelerator depression ΔAccsuggests that the driver demands a higher acceleration. Setting theshorter cancellation time t enables the torque restriction with thetorque restoration limit δ1 to be cancelled in a shorter time period, inresponse to the driver's high acceleration demand. After setting thecancellation time t, the CPU 42 waits until elapse of the setcancellation time t (step S216). When the cancellation time t haselapsed, the CPU 42 subsequently sets a cancellation increment D1 of acancellation rate Δδ1 for canceling the torque restoration limit δ1,based on the calculated additional accelerator depression ΔAcc and theinput skid-state accelerator opening Accslip (step S218). The CPU 42then increments the cancellation rate Δδ1 by the set cancellationincrement D1 to update the cancellation rate Δδ1 (step S219) and exitsfrom this torque restoration limit δ1 cancellation routine. A concreteprocedure of setting the cancellation increment D1 in this embodimentstores in advance variations in cancellation increment D1 against theadditional accelerator depression ΔAcc and the skid-state acceleratoropening Accslip as a map in the ROM 44 and reads the cancellationincrement D1 corresponding to the given additional acceleratordepression ΔAcc and the given skid-state accelerator opening Accslipfrom the map. One example of this map is shown in FIG. 11. As shown inFIG. 11, a greater value is set to the cancellation increment D1 with anincrease in additional accelerator depression ΔAcc. The greateradditional accelerator depression ΔAcc suggests that the driver demandsa higher acceleration. Setting the greater cancellation increment D1enables the torque restriction with the torque restoration limit δ1 tobe cancelled by a greater degree, in response to the driver's highacceleration demand. The concrete process of setting the cancellationrate Δδ1 writes the value of the cancellation rate Δδ1 into a specificarea of the RAM 46.

Referring back to the routine of FIG. 7, in the event of detection of acancellation request, the CPU 42 subtracts the cancellation rate Δδ1from the torque restoration limit δ1, which is input at step S170, tocancel the torque restoration limit δ1 (step S176). In the event of nodetection of a cancellation request, on the other hand, the torquerestoration limit δ1 is not cancelled. The torque restoration limit δ1is not cancelled until elapse of the cancellation time t at step S216 inthe routine of FIG. 9 after the start of the skid convergence statecontrol routine. The angular acceleration α calculated at step S104 inthe routine of FIG. 2 is then compared with the sum of the torquerestoration limit δ1 and the torque restriction rate δsafe (step S178).In this cycle, it is assumed that no skid reoccurs. The torquerestriction rate δsafe is thus equal to 0, and the angular accelerationα is not greater than the sum of the torque restoration limit δ1 and thetorque restriction rate δsafe (=0). The CPU 42 accordingly refers to themap of FIG. 6 and sets the maximum torque Tmax as an upper limit oftorque output from the motor 12 corresponding to the torque restorationlimit δ1 (step S180).

After setting the maximum torque Tmax, the motor torque demand Tm* iscompared with the preset maximum torque Tmax (step S184). When the motortorque demand Tm* exceeds the maximum torque Tmax, the motor torquedemand Tm* is limited to the maximum torque Tmax (step S186). The CPU 42then sets the motor torque demand Tm* to a target torque and drives andcontrols the motor 12 to output a torque corresponding to the targettorque Tm* (step S188). The torque control of the motor 12 based on thetorque restoration limit δ1, which is set corresponding to the timeintegration of the angular acceleration α, ensures restoration of therestricted torque to an adequate level in response to convergence of askid according to the current skid state. Under the condition of a largetime integration of the angular acceleration α, which suggests a highpotential for occurrence of another skid, the torque restoration levelis set low in response to convergence of a skid. Under the condition ofs small time integration of the angular acceleration α, which suggests alow potential for occurrence of another skid, on the contrary, thetorque restoration level is set high to effectively prevent theoccurrence of another skid without excessive torque restriction. Afterthe drive control of the motor 12, the CPU 42 determines whether thetorque restoration limit δ1 is not higher than 0, that is, whether thetorque restoration limit δ1 is completely cancelled (step S190). In thecase of complete cancellation, both the skid occurrence flag F1 and theskid convergence flag F2 are reset to zero (step S192). The program thenterminates the skid convergence state control routine.

The above description regards the skid convergence state control on theassumption of no reoccurrence of a skid. The following description is onthe assumption of reoccurrence of a skid during the repeated executionof the skid convergence state control routine. In the event ofreoccurrence of a skid, the torque restriction is implemented again withthe setting of the torque restriction rate δsafe. The torque restrictionrate δsafe is set according to a torque restriction rate δsafe settingand cancellation routine shown in FIG. 12. This routine is executedrepeatedly at preset time intervals (for example, at every 8 msec)during the repeated execution of the skid convergence state controlroutine of FIG. 7, that is, for a time period between the time ofsetting the skid convergence flag F2 to 1 and the time of resetting theskid convergence flag F2 to 0.

When the torque restriction rate δsafe setting and cancellation routinestarts, the CPU 42 of the electronic control unit 40 first inputs therotation speed Nm of the motor 12 (step S220) and calculates the angularacceleration α from the input rotation speed Nm (step S222). The CPU 42then determines whether the calculated angular acceleration α exceedsthe preset threshold value αslip, that is, detects reoccurrence ornon-reoccurrence of a skid (step S224). In response to detection of noreoccurrence of a skid, the CPU 42 immediately exits from this routinewithout any further processing. In response to detection of reoccurrenceof a skid, on the other hand, the CPU 42 subsequently determines whethera differential dα/dt of the angular acceleration α is close to 0, thatis, whether the angular acceleration α has reached a peak (step S226).When it is determined that the angular acceleration α has reached apeak, the current value of the angular acceleration α is set to a peakvalue αpeak (step S228). When it is determined that the angularacceleration α has not yet reached a peak, on the other hand, the CPU 42immediately exits from this routine without any further processing.

The CPU 42 then sets the torque restriction rate δsafe for reduction ofthe reoccurring skid, based on the peak value αpeak (step S230). Aconcrete procedure of setting the torque restriction rate δsafe in thisembodiment stores in advance a variation in torque restriction rateδsafe against the peak value αpeak as a map in the ROM 44 and reads thetorque restriction rate δsafe corresponding to the given peak valueαpeak. One example of this map is shown in FIG. 13. As shown in FIG. 13,this map sets a greater value to the torque restriction rate δsafe withan increase in peak value αpeak of the angular acceleration α. Thetorque restriction rate δsafe is set basically to reduce another skid,which is caused by forced cancellation of the torque restoration rate δ1in response to the driver's additional depression of the acceleratorpedal 33. The procedure of this embodiment regulates the torquerestriction rate δsafe to a sufficient value for effectively preventingan excess skid of the drive wheels 18 a and 18 b, which may lead to anunstable state of the vehicle 10.

After setting the torque restriction rate δsafe, the CPU 42 inputs theskid-state accelerator opening Accslip and the accelerator opening Acc(step S232) and calculates the additional accelerator depression ΔAcc(=Acc−Accslip) (step S234). The CPU 42 sets a cancellation time t of thetorque restriction rate δsafe, based on the calculated additionalaccelerator depression ΔAcc and the input skid-state accelerator openingAccslip (step S236) and waits until elapse of the set cancellation timet (step S238). A map similar to the map of FIG. 10 used for theprocessing of step S214 in the torque restoration limit δ1 cancellationroutine of FIG. 9 is basically used to set the cancellation time t.Since the torque restriction rate δsafe is set to prevent an excessskid, it is desirable that the cancellation time t of the torquerestriction rate δsafe is shorter than the cancellation time t of thetorque restoration limit δ1. After elapse of the set cancellation timet, the CPU 42 fully cancels the torque restriction rate δsafe (stepS240) and exits from this routine. This procedure cancels the torquerestriction rate δsafe all at once. One modified procedure may graduallycancel the torque restriction rate δsafe with elapse of time. Theconcrete process of setting and canceling the torque restriction rateδsafe writes the value of the torque restriction rate δsafe into aspecific area of the RAM 46. The value of the torque restriction rateδsafe written in the specific area of the RAM 46 is read to be processedin the skid convergence state control routine of FIG. 7. The flow of theroutine of FIG. 7 in response to detection of reoccurrence of a skid isdescribed below, while description of the overlapped portion with theflow in response to detection of no reoccurrence of a skid is omitted.

In the event of reoccurrence of a skid, the skid convergence statecontrol routine of FIG. 7 is executed for a time period between the timeof setting the torque restriction rate δsafe and the time of cancelingthe torque restriction rate δsafe. The CPU 42 inputs the set torquerestriction rate δsafe (step S170) and sets the maximum torque Tmax,based on the sum of the torque restoration limit δ1 and the torquerestriction rate δsafe (δ1+δsafe) (step S182). In this skid reoccurringstate, the driver's additional depression of the accelerator pedal 33partly cancels the torque restoration limit δ1, and the motor 12 iscontrolled with the maximum torque Tmax, which has been set only basedon the torque restoration limit δ1. The control procedure accordinglyrefers to the map of FIG. 6 and sets the maximum torque Tmaxcorresponding to the sum of the torque restoration limit δ1 and thetorque restriction rate δsafe and restricts the torque output from themotor 12. This effectively prevents reoccurrence of an excess skid. Themaximum torque Tmax is set in this manner (step S180) in the case ofreoccurrence of a relatively light skid when the angular acceleration αis not greater than the sum of the torque restoration limit δ1 and thetorque restriction rate δsafe at step S178 in FIG. 7. In the case ofreoccurrence of a relatively heavy skid when the angular acceleration αis greater than the sum of the torque restoration limit δ1 and thetorque restriction rate δsafe, the control procedure sets the maximumtorque Tmax based on the sum of the torque restoration limit δ1, thetorque restriction rate δsafe, and the angular acceleration α(δ1+δsafe+α) (step S182) and controls the operation of the motor 12 withthe more restricted maximum torque Tmax. One modified procedure may setthe maximum torque Tmax based on the sum of the torque restoration limitδ1 and the torque restriction rate δsafe, regardless of the magnitude ofa reoccurring skid.

FIG. 14 shows a process of setting the maximum torque Tm*. In responseto detection of a skid when the angular acceleration α of the rotatingshaft of the motor 12 exceeds the preset threshold value αslip, thecontrol procedure gradually decreases the maximum torque Tmax with avariation in angular acceleration α according to the map of FIG. 6. Whenthe angular acceleration α reaches a peak, the torque level isrestricted to the maximum torque Tmax (=value T1) corresponding to thepeak value αpeak (see FIG. 14( a)). The maximum torque Tmax is kept tothe value T1 until determination of convergence of the skid based on anegative level of the angular acceleration α. In response todetermination of convergence of the skid, the torque level is restoredto the maximum torque Tmax (=value T2) corresponding to the torquerestoration limit δ1, which is set according to the time integration ofthe angular acceleration α (that is, the skid state), irrespective ofthe current value of the angular acceleration α (see FIG. 14( b)). Thelimitation of torque restoration with the torque restoration limit δ1effectively prevents reoccurrence of a skid. After elapse of the presetcancellation time according to the driver's additional depression ΔAccof the accelerator pedal 33, the torque restoration limit δ1 iscancelled by the cancellation rate corresponding to the additionalaccelerator depression ΔAcc. The torque level is then restored to themaximum torque Tmax (=value T3) corresponding to the updated torquerestoration limit δ1 (see FIG. 14( c)). In the event of reoccurrence ofa skid due to the torque restoration, the torque level is restrictedagain to the maximum torque Tmax (=value T4) corresponding to the sum ofthe updated torque restoration limit δ1 and the peak value αpeak of theangular acceleration α increasing in the skid reoccurring state (seeFIG. 14( d)). In this state, the torque restriction rate δsafe is setcorresponding to the peak value αpeak of the angular acceleration α.Even in the event of a decrease in angular acceleration α due to anothertorque restriction, the torque restoration level is limited again to themaximum torque Tmax (=value T5) corresponding to the sum of the torquerestoration limit δ1 and the torque restriction rate δsafe (see FIG. 14(e)). The torque restriction rate δsafe is cancelled according to theadditional accelerator depression ΔAcc after elapse of the presetcancellation time. The torque level is accordingly restored to themaximum torque Tmax (=value T6) corresponding to only the torquerestoration limit δ1 (see FIG. 14( f)).

As described above, the motor control apparatus 20 of the embodimentrestricts the torque output from the motor 12 in response to theoccurrence of a skid due to wheelspin of the drive wheels 18 a and 18 b.In the event of reduction of the skid, the motor control apparatus 20varies the degree of cancellation of the torque restriction (thecancellation rate and the cancellation time) according to the driver'sadditional depression ΔAcc of the accelerator pedal 33. The controlprocedure of this embodiment sets a greater value to the cancellationrate of the torque restriction and a smaller value to the cancellationtime with an increase in additional depression ΔAcc of the acceleratorpedal 33. Such setting ensures a certain level of response to thedriver's acceleration demand, while effectively reducing the skid of thedrive wheels 18 a and 18 b. This arrangement enhances the drivability inthe skid control. In the event of reoccurrence of a skid by cancellationof the torque restriction in response to the driver's additionaldepression of the accelerator pedal 33, the control procedure controlsthe motor 12 to prevent an excess level of the reoccurring skid. Thisarrangement thus makes the driver feel the reoccurrence of a skid andrelease the accelerator pedal 33, while preventing the excess level ofthe reoccurring skid, which may lead to the unstable state of thevehicle 10.

In the event of reoccurrence of a skid, that is, when the angularacceleration α exceeds the preset threshold value αslip again duringrepeated execution of the skid convergence state control routine of FIG.7, the motor control apparatus 20 of the embodiment sets the torquerestriction rate δsafe according to the peak value αpeak of the angularacceleration α and restricts the torque level again with the set torquerestriction rate δsafe to prevent an excess level of the reoccurringskid. One modified procedure may alternatively execute the skidoccurring state control routine of FIG. 5 in response to reoccurrence ofa skid. This modified procedure resets the skid convergence flag F2 from1 to 0 when it is determined at step S130 that the angular accelerationα exceeds the preset threshold value αslip in the skid statedetermination routine of FIG. 4. This triggers the skid occurring statecontrol routine, instead of the skid convergence state control routine,since the skid occurrence flag F1 is equal to 1 and the skid convergenceflag F2 is equal to 0. This modified procedure naturally does notrequire the series of processing with respect to the torque restrictionrate δsafe.

A motor control apparatus of a second embodiment is discussed below. Themotor control apparatus of the second embodiment has the same hardwareconfiguration as that of the motor control apparatus 20 of the firstembodiment. The only difference is series of processing executed by theelectronic control unit. The hardware configuration of the motor controlapparatus of the second embodiment is thus not specifically describedhere. The motor control apparatus 20 of the first embodiment detects askid based on a variation in angular acceleration α and controls theoperation of the motor 12 in response to detection of the skid. Themotor control apparatus of the second embodiment, on the other hand,detects a skid based on a variation in difference between the wheelspeed Vf of the drive wheels and the wheel speed Vr of the driven wheels(that is, wheel speed difference ΔV) and controls the operation of themotor in response to detection of the skid. Determination of the skidstate based on the wheel speed difference ΔV follows a skid statedetermination routine shown in FIG. 15.

When the skid state determination routine of FIG. 15 starts, the CPU ofthe electronic control unit first determines whether the wheel speeddifference ΔV exceeds a preset threshold value Vslip (step S270). Whenthe wheel speed difference ΔV exceeds the preset threshold value Vslip,the CPU detects the occurrence of a skid and sets a skid occurrence flagF3 to 1 (step S272) and resets a skid convergence flag F4 to 0 (stepS273), before exiting from this routine. When the wheel speed differenceΔV does not exceed the preset threshold value Vslip, on the other hand,the CPU subsequently determines whether the skid occurrence flag F3 isequal to 1 (step S274). When the skid occurrence flag F3 is equal to 1,the CPU determines convergence of the skid and sets the skid convergenceflag F4 to 1 (step S276), before exiting from this routine. When theskid occurrence flag F3 is not equal to 1, on the contrary, the CPUresets both the flags F3 and F4 to 0 (step S278) and terminates thisroutine.

The motor control procedure based on the determined skid state executesgrip state control when both the flags F3 and F4 are equal to 0, skidoccurring state control when the flag F3 is equal to 1 and the flag F4is equal to 0, and skid convergence state control when both the flags F3and F4 are equal to 1. These controls are described in detail. The gripstate control is identical with the grip state control executed by themotor control apparatus 20 of the first embodiment and is thus notspecifically described here.

The skid occurring state control drives and controls the motor to lowerthe wheel speed difference ΔV, which was increased by the occurrence ofa skid, and follows a skid occurring state control routine of FIG. 16.When the skid occurring state control routine starts, the CPU of theelectronic control unit first inputs a torque restriction rate δ2 (stepS260). The torque restriction rate δ2 is a parameter used to set themaximum torque Tmax of the motor for elimination of a skid. The torquerestriction rate δ2 is set according to a torque restriction rate δ2setting routine shown in FIG. 17 as discussed below. The torquerestriction rate δ2 setting routine of FIG. 17 is executed repeatedly atpreset time intervals (for example, at every 8 msec) for a time periodbetween the time of setting the skid occurrence flag F3 from 0 to 1 atstep S272 in the skid state determination routine of FIG. 15 and thetime of setting the skid convergence flag F4 from 0 to 1. The torquerestriction rate δ2 setting routine first inputs the wheel speeds Vf andVr (step S290), calculates the wheel speed difference ΔV as a differencebetween the input wheel speeds Vf and Vr (step S292), and integrates thecalculated wheel speed difference ΔV to give a time integration Vintthereof over an integration interval since the wheel speed difference ΔVexceeded the preset threshold value Vslip (step S294). In thisembodiment, the time integration Vint of the wheel speed difference ΔVis given by Equation (2) below, where Δt denotes the execution timeinterval of this routine:Vint←Vint+(ΔV−Vslip)·Δt   (2)

The torque restriction rate δ2 is set by multiplying the timeintegration Vint of the wheel speed difference ΔV by a predeterminedcoefficient k2 (step S296). The torque restriction rate δ2 settingroutine is here terminated. This routine calculates the torquerestriction rate δ2 by multiplication of the predetermined coefficientk2. One modified procedure may prepare in advance a map representing avariation in torque restriction rate δ2 against the time integrationVint and read the torque restriction rate δ2 corresponding to the giventime integration Vint from the map. The set torque restriction rate δ2is successively written into a specific area of the RAM 46 to be updatedand is input in the routine of FIG. 16. The procedure of this embodimentsets the torque restriction rate δ2 corresponding to the timeintegration of the wheel speed difference ΔV. The torque restrictionrate δ2 may otherwise be set corresponding to the value of the wheelspeed difference ΔV or may be fixed to a preset value regardless of thevalue of the wheel speed difference ΔV.

Referring back to the routine of FIG. 16, after input of the torquerestriction rate δ2, the maximum torque Tmax as the upper limit oftorque output from the motor 12 is set corresponding to the input torquerestriction rate δ2 by referring to the map of FIG. 6 (step S282). Aftersetting the maximum torque Tmax, a motor torque demand Tm* is comparedwith the maximum torque Tmax (step S284). When the motor torque demandTm* exceeds the maximum torque Tmax, the motor torque demand Tm* islimited to the maximum torque Tmax (step S286). The CPU then sets themotor torque demand Tm* to a target torque and drives and controls themotor 12 to output a torque corresponding to the target torque Tm* (stepS288), before exiting from this skid occurring state control routine.The torque output from the motor 12 in the occurrence of a skid islimited to a lower level (that is, the maximum torque Tmax correspondingto the torque restriction rate δ2 [rpm/8 msec] in the map of FIG. 6) forimmediate reduction of the skid. This limitation effectively reduces theskid.

The skid convergence state control drives and controls the motor torestore the torque level limited in response to the decreasing wheelspeed difference ΔV by the skid occurring state control, and follows askid convergence state control routine of FIG. 18. When the skidconvergence state control routine starts, the CPU of the electroniccontrol unit first inputs the last setting of the torque restrictionrate δ2, which has been set in the last cycle of the repeatedly executedtorque restriction rate δ2 setting routine of FIG. 17 (that is,immediately before the skid convergence flag F4 is set from 0 to 1)(step S300). The CPU 42 receives a cancellation request of the inputtorque restriction rate δ2 if any (step S302) and determines whether thecancellation request has been entered (step S304). The cancellationrequest of the torque restriction rate δ2 is entered according to atorque restriction rate δ2 cancellation routine of FIG. 19. This torquerestriction rate δ2 cancellation routine is basically similar to thetorque restoration limit δ1 cancellation routine of FIG. 9 and isexecuted repeatedly at preset time intervals (for example, at every 8msec) during execution of the skid convergence state control routine ofFIG. 18. The torque restriction rate δ2 cancellation routine firstinputs the skid-state accelerator opening Accslip and the acceleratoropening Acc (step S320), calculates their difference as the additionalaccelerator depression ΔAcc (step S322), and sets a cancellation time tof the torque restriction rate δ2, based on the calculated additionalaccelerator depression ΔAcc and the input skid-state accelerator openingAccslip (step S324). The cancellation time t is set according to a maphaving the similar characteristics to those of the map of FIG. 10. Aftersetting the cancellation time t, the routine waits until elapse of theset cancellation time t (step S2326). When the cancellation time t haselapsed, the routine subsequently sets a cancellation increment D2 of acancellation rate Δδ2 for canceling the torque restriction rate δ2,based on the calculated additional accelerator depression ΔAcc and theinput skid-state accelerator opening Accslip (step S328). The routinethen increments the cancellation rate Δδ2 by the set cancellationincrement D2 to update the cancellation rate Δδ2 (step S330) and isterminated. The cancellation increment D2 is set according to a maphaving the similar characteristics to those of the map of FIG. 11. Thecancellation rate Δδ2 is successively written into a specific area ofthe RAM 46 to bed updated and is subjected to the processing routine ofFIG. 18.

Referring back to the skid convergence state control routine of FIG. 18,in the event of detection of a cancellation request (that is, when thecancellation rate Δδ2 is not equal to 0), the CPU subtracts thecancellation rate Δδ2 from the torque restriction rate δ2, which isinput at step S230, to cancel the torque restriction rate δ2 (stepS306). In the event of no detection of a cancellation request, on theother hand, the torque restriction rate δ2 is not cancelled. The torquerestriction rate δ2 is not cancelled until elapse of the cancellationtime t at step S326 in the routine of FIG. 19 after the start of theskid convergence state control routine. The CPU then refers to the mapof FIG. 6 and sets the maximum torque Tmax as an upper limit of torqueoutput from the motor 12 corresponding to the torque restriction rate δ2(step S308). After setting the maximum torque Tmax, the motor torquedemand Tm* is compared with the preset maximum torque Tmax (step S310).When the motor torque demand Tm* exceeds the maximum torque Tmax, themotor torque demand Tm* is limited to the maximum torque Tmax (stepS312). The CPU then sets the motor torque demand Tm* to a target torqueand drives and controls the motor 12 to output a torque corresponding tothe target torque Tm* (step S314). The CPU subsequently determineswhether the torque restriction rate δ2 is not higher than 0, that is,whether the torque restriction rate δ2 is completely cancelled (stepS316). In the case of complete cancellation, both the skid occurrenceflag F3 and the skid convergence flag F4 are reset to zero (step S318).The skid convergence state control routine is here terminated. In theevent of reoccurrence of a skid (when the wheel speed difference ΔVexceeds again the preset threshold value Vslip) during execution of theskid convergence state control routine of FIG. 18 after convergence of askid (after the wheel speed difference ΔV becomes lower than the presetthreshold value Vslip), the skid convergence flag F4 is reset from 1 to0 at step S273 in the skid state determination routine of FIG. 15. Thistriggers the skid occurring state control routine of FIG. 18 to reducethe reoccurring skid.

FIG. 20 shows a process of setting the maximum torque Tmax. As shown inFIG. 20, in response to detection of a skid when the wheel speeddifference ΔV exceeds the preset threshold value Vslip, the controlprocedure gradually increases the torque restriction rate δ2 regardlessof the angular acceleration α until the wheel speed difference ΔVbecomes lower than the preset threshold value Vslip. With the increasein torque restriction rate δ2, the maximum torque Tmax graduallydecreases to restrict the torque level (see FIGS. 20( a) through 20(c)).The increase of the torque restriction rate δ2 is set according to thetime integration of the wheel speed difference ΔV since the time whenthe wheel speed difference ΔV exceeded the preset threshold value Vlip.Under the condition that the wheel speed difference ΔV becomes lowerthan the preset threshold value Vlip, after elapse of the presetcancellation time according to the driver's additional depression ΔAccof the accelerator pedal 33, the torque restriction rate δ2 is cancelledby the cancellation rate Δδ2 set corresponding to the additionaldepression ΔAcc of the accelerator pedal 33. The torque level is thenrestored to the maximum torque Tmax (=value T4) corresponding to theupdated torque restriction rate δ2 (see FIG. 20( d)). The controlprocedure then cancels the torque restriction rate δ2 in a stepwisemanner to gradually restore the torque level.

As described above, like the motor control apparatus 20 of the firstembodiment, the motor control apparatus of the second embodiment ensuresa certain level of response to the driver's acceleration demand, whileeffectively reducing the skid of the drive wheels 18 a and 18 b. Thisarrangement enhances the drivability in the skid control.

The motor control apparatus of the second embodiment detects a skidbased on the variation in wheel speed difference ΔV, independently ofdetection of a skid based on the variation in angular acceleration α bythe motor control apparatus 20 of the first embodiment. The detection ofa skid based on the variation in wheel speed difference ΔV may beexecuted only in the case of no detection of a skid based on thevariation in angular acceleration α or may be executed in parallel withdetection of a skid based on the variation in angular acceleration α.Such modifications advantageously succeed in detecting a minor skid,which is undetectable based on the variation in angular acceleration α,based on the variation in wheel speed difference ΔV. In the lattermodification, in the event of detection of a skid by both the skiddetection based on the angular acceleration α and the skid detectionbased on the wheel speed difference ΔV, the skid occurring state controlmay refer to the map of FIG. 6, set the maximum torque Tmaxcorresponding to the sum of the peak value αpeak [rpm/8 msec] of theangular acceleration α set at step S152 in the skid occurring statecontrol routine of FIG. 5 and the torque restriction rate δ2 [rpm/8msec] input at step S280 in the skid occurring state control routine ofFIG. 16 (Tmax←g(αpeak+δ2)), and control the motor 12 with the setting ofthe maximum torque Tmax. The skid occurring state control mayalternatively set the maximum torque Tmax corresponding to the greaterbetween the peak value αpeak of the angular acceleration α and thetorque restriction rate δ2 and control the motor 12 with the setting ofthe maximum torque Tmax. Similarly the skid convergence state controlmay refer to the map of FIG. 6, set the maximum torque Tmaxcorresponding to the grand total of the sum (δ1+δsafe) of the torquerestoration limit δ1 set at step S176 (or input at step S170) and thetorque restriction rate δsafe input at step S170 in the skid convergencestate control routine of FIG. 7, or the sum (δ1+δsafe+α) of (δ1+δsafe)and the angular acceleration α when the angular acceleration α exceeds(δ1+δsafe), and the torque restriction rate δ2 set at step S306 (orinput at step S300) in the skid convergence state control routine ofFIG. 18 (Tmax←g(δ1+δsafe+δ2) or g(δ1+δsafe+δ2+α)), and control the motor12 with the setting of the maximum torque Tmax. The skid convergencestate control may alternatively set the maximum torque Tmaxcorresponding to the greater between (δ1+δsafe) and δ2 or between(δ1+δsafe+α) and δ2 and control the motor 12 with the setting of themaximum torque Tmax.

The embodiments described above regard control of the motor 12, which ismounted on the vehicle 10 and is mechanically connected with the driveshaft linked to the drive wheels 18 a and 18 b to directly output powerto the drive shaft. The technique of the invention is applicable to avehicle of any other structure with a motor that is capable of directlyoutputting power to a drive shaft. For example, one possible applicationof the invention is a series hybrid vehicle including an engine, agenerator that is linked to an output shaft of the engine, a batterythat is charged with electric power generated by the generator, and amotor that is mechanically connected with a drive shaft linked to drivewheels and is driven with a supply of electric power from the battery.Another possible application of the invention is a mechanicaldistribution-type hybrid vehicle 110 including an engine 111, aplanetary gear 117 that is connected with the engine 111, a motor 113that is connected with the planetary gear 117 and is capable ofgenerating electric power, and a motor 112 that is also connected withthe planetary gear 117 and is mechanically connected with a drive shaftlinked to drive wheels to directly output power to the drive shaft, asshown in FIG. 21. Still another possible application of the invention isan electrical distribution-type hybrid vehicle 210 including a motor 212that has an inner rotor 213 a connected with an output shaft of anengine 211 and an outer rotor 213 b connected with a drive shaft linkedto drive wheels 218 a and 218 b and relatively rotates throughelectromagnetic interactions between the inner rotor 213 a and the outerrotor 213 b and a motor 212 that is mechanically connected with thedrive shaft to directly output power to the drive shaft, as shown inFIG. 22. Another possible application of the invention is a hybridvehicle 310 including an engine 311 that is connected with a drive shaftlinked to drive wheels 318 a and 318 b via a transmission 314 (forexample, a continuous variable transmission or an automatictransmission) and a motor 312 that is placed after the engine 311 and isconnected with the drive shaft via the transmission 314 (or a motor thatis directly connected with the drive shaft), as shown in FIG. 23. In theevent of the occurrence of a skid on drive wheels, the torque controlmainly controls the motor mechanically connected with the drive shaft,because of its high torque output response. The control of this motormay be combined with control of the other motor or with control of theengine.

The embodiments and their modified examples discussed above are to beconsidered in all aspects as illustrative and not restrictive. There maybe many other modifications, changes, and alterations without departingfrom the scope or spirit of the main characteristics of the presentinvention.

INDUSTRIAL APPLICABILITY

The technique of the invention is effectively applied to automobile andtrain-related industries.

1. A motor control apparatus that controls a motor, which is mounted ona vehicle and outputs power to a drive shaft linked to drive wheels,said motor control apparatus comprising: a skid detection module thatdetects a skid due to wheel spin of the drive wheels; a torquerestriction control module that, in response to detection of a skid bysaid skid detection module, sets torque restriction for reduction of theskid and controls said motor under the torque restriction; and a torquerestriction cancellation control module that, in response to at least areducing tendency of the skid, cancels the torque restriction, which isset by said torque restriction control module, to a specific degreecorresponding to a variation in a driver's accelerator operation, andcontrols said motor under at least partly cancelled torque restrictions,wherein the variation in driver's accelerator operation represents arate of change relative to a reference accelerator operation at a timeof detection of a skid by said skid detection module.
 2. A motor controlapparatus in accordance with claim 1, wherein said torque restrictioncancellation control module cancels the torque restriction in a stepwisemanner with elapse of time.
 3. A motor control apparatus in accordancewith claim 2, wherein said torque restriction cancellation controlmodule controls the motor with a tendency of increasing a cancellationrate of the torque restriction with an increase in driver's additionaldepression of an accelerator pedal as the variation in driver'saccelerator operation.
 4. A motor control apparatus in accordance withclaim 2, wherein said torque restriction cancellation control modulecontrols the motor with a tendency of shortening a cancellation time ofthe torque restriction with an increase in driver's additionaldepression of an accelerator pedal as the variation in driver'saccelerator operation.
 5. A motor control apparatus in accordance withclaim 1, said motor control apparatus further comprising: an angularacceleration measurement module that measures an angular acceleration ofeither of the drive shaft and a rotating shaft of the motor, whereinsaid skid detection module detects a skid, based on a variation inmeasured angular acceleration, and said torque restriction controlmodule, in response to detection of a skid, changes a degree of thetorque restriction corresponding to the angular acceleration measured bysaid angular acceleration measurement module and controls the motorunder the changed degree of the torque restriction.
 6. A motor controlapparatus in accordance with claim 1, wherein said vehicle has drivenwheels that are driven by the drive wheels, said motor control apparatusfurther comprising: a drive wheel rotation speed measurement module thatmeasures a rotation speed of the drive wheels; and a driven wheelrotation speed measurement module that measures a rotation speed of thedriven wheels; wherein said skid detection module detects a skid, basedon a rotation speed difference between the rotation speed of the drivewheels measured by said drive wheel rotation speed measurement moduleand the rotation speed of the driven wheels measured by said drivenwheel rotation speed measurement module, and said torque restrictioncontrol module, in response to detection of a skid, changes a degree ofthe torque restriction corresponding to the rotation speed differenceand controls the motor under the changed degree of the torquerestriction.
 7. A motor control apparatus in accordance with claim 1,said motor control apparatus further comprising: a torque re-restrictioncontrol module that, in response to detection of another skid by saidskid detection module under control of the motor by said torquerestriction cancellation control module, sets torque re-restriction forreduction of the another skid and controls the motor under the torquere-restriction.
 8. A motor control apparatus in accordance with claim 7,said motor control apparatus further comprising: an angular accelerationmeasurement module that measures an angular acceleration of either ofthe drive shaft and a rotating shaft of the motor, wherein said skiddetection module detects a skid, based on a variation in measuredangular acceleration, and said torque re-restriction control module, inresponse to detection of another skid by said skid detection module,changes a degree of the torque re-restriction corresponding to a peakvalue of the angular acceleration measured by said angular accelerationmeasurement module and controls the motor under the changed degree ofthe torque re-restriction.
 9. A motor control apparatus in accordancewith claim 7, said motor control apparatus further comprising: a torquerestriction re-cancellation control module that cancels the torquere-restriction set by said torque re-restriction control module afterelapse of a preset time period corresponding to a variation in driver'saccelerator opening, regardless of state of the another skid, andcontrols the motor under the cancelled torque re-restriction.
 10. Avehicle equipped with a motor and a motor control apparatus, wherein themotor control apparatus controls the motor, which is mounted on thevehicle and outputs power to a drive shaft linked to drive wheels, saidmotor control apparatus comprising: a skid detection module that detectsa skid due to wheel spin of the drive wheels; a torque restrictioncontrol module that, in response to detection of a skid by said skiddetection module, sets torque restriction for reduction of the skid andcontrols said motor under the torque restriction; and a torquerestriction cancellation control module that, in response to at least areducing tendency of the skid, cancels the torque restriction, which isset by said torque restriction control module, to a specific degreecorresponding to a variation in a driver's accelerator operation, andcontrols said motor under at least partly cancelled torque restriction,wherein the variation in driver's accelerator operation represents arate of change relative to a reference accelerator operation at a timeof detection of a skid by said skid detection module.
 11. A motorcontrol method that controls a motor, which is mounted on a vehicle andoutputs power to a drive shaft linked to drive wheels, said motorcontrol method comprising the steps of: (a) detecting a skid due towheelspin of the drive wheels; (b) in response to detection of a skid bysaid step (a), setting torque restriction for reduction of the skid andcontrolling said motor under the torque restriction; and (c) in responseto at least a reducing tendency of the skid, canceling the torquerestriction, which is set by said step (b), to a specific degreecorresponding to a variation in a driver's accelerator operation, andcontrolling said motor under at least partly cancelled torquerestriction, wherein the variation in driver's accelerator operationrepresents a rate of change relative to a reference acceleratoroperation at a time of detection of a skid by said step (a).
 12. A motorcontrol method in accordance with claim 11, wherein said step (c)cancels the torque restriction in a stepwise manner with elapse of time.13. A motor control method in accordance with claim 12, wherein saidstep (c) controls the motor with a tendency of increasing a cancellationrate of the torque restriction with an increase in driver's additionaldepression of an accelerator pedal as the variation in driver'saccelerator operation.
 14. A motor control method in accordance withclaim 12, wherein said step (c) controls the motor with a tendency ofshortening a cancellation time of the torque restriction with anincrease in driver's additional depression of an accelerator pedal asthe variation in driver s accelerator operation.
 15. A motor controlapparatus in accordance with claim 3, wherein said torque restrictioncancellation control module controls the motor with a tendency ofshortening a cancellation time of the torque restriction with anincrease in driver's additional depression of an accelerator pedal asthe variation in driver's accelerator operation.
 16. A motor controlmethod in accordance with claim 13, wherein said step (c) controls themotor with a tendency of shortening a cancellation time of the torquerestriction with an increase in driver's additional depression of anaccelerator pedal as the variation in driver's accelerator operation.